R.m.s. voltage regulator



A ril 2, 1963 ca. F. MORRIS ETAL- R.M.S. VOLTAGE REGULATOR Filed Feb. 29, 1960 SW w w M N n III m w Ill. N E H I o m m z L H C United States Patent 3,084,325 R.M.S. VOLTAGE REGULATOR George F. Morris and Charles H. Reynolds, Rochester, N.Y., assignors to General Dynamics Corporation, Rochester, N.Y., a corporation of Delaware Filed Feb. 29, 1960, Ser. No. 11,582 1 Claim. (Cl. 323-38) The present invention relates generally to a new and improved voltage regulating circuit and is more particularly concerned with a circuit having a regulated R.M.S. output voltage while employing only very small, light components arranged to conserve power and to provide very fast response to variations in the amplitude of the input voltage.

In a number of applications, for example, in controlling the filament temperatures of one or more tubes connected across an A.C. line, it is desirable to maintain sub-stantially constant R.M.S. voltage. Prior to the presentinvention, it has been customary to use ballast tubes or magnetic amplifiers to effect the desired control. However, neither of these devices is entirely satisfactory, since the ballast tubes are wasteful of power while the use of a magnetic amplifier requires a heavy, space consuming device usually of a size commensurate with that of the transformer whose output voltage is being regulated. The primary object of the present invention is, therefore, to provide a new and improved voltage regulating circuit which avoids all of the aforementioned disadvantages of prior art circuits.

The invention has 'for another object the prow'sion of a voltage regulating circuit of the character described above employing only very small, light and inexpensive components.

A further object of the invention is to provide a circuit delivering a regulated R.M.S. output voltage from an A.C. input subject to voltage excursions with the circuit being characterized by a very fast response to the input voltage variations.

In accordance with the present invention the foregoing and other objects are-realized by the provision of a circuit for providing aregualted R.M.S. voltage output in response to an A.C. input signal which may vary in amplitude. The circuit includes an input transformer having its primary excited by the A.C. input signal and having its secondary regulated to provide nearly constant R.M.S. output voltage. To provide the regulation the transformer secondary circuit includes a silicon controlled rectifier which effectively deletes a portion of the secondary signal. The amount of signal deleted is controlled by applying to the gate electrode of the controlled rectifier a control voltage developed 'by an R.M.S. sensing device connected across the output. The latter sensing device, which preferably comprises a temperature limited diode, is sensitive to small changes in R.M.S. voltage applied across its filament and it functions to develop a D'.C. control signal which is amplified and supplied to the gate electrode of the controlled rectifier. Thus, if the output voltage tends to increase, the sensing device and its associated amplifier apply a signal to the gate electrode which causes the controlled rectifier to delete a greater portion of the secondary voltage. A decrease in the output voltage, on the other hand, causes the control rectifier to delete a smaller portion of the secondary voltage. Through the circuit described, the R.M.S. output voltage can be maintained substantially constant despite very wide variations in the input voltage.

The invention, both as to its organization and manner of operation together with further objects and advantages, will be understood by reference to the following description taken in conjunction with the accompanying drawing wherein the single FIGURE illustrates a regulating 3,084,325 Patented Apr. 2, 1963 circuit 10 characterized by the features of the present invention. This circuit is excited by A.C. input signals supplied from a suitable source 11 across the primary winding 12 of a conventional input transformer 13. The A.C. input signal is sinusoidal in form but is not regulated in amplitude and, hence, may be subject to appreciable voltage excursions. It is the purpose of the circuit 10 to provide a substantially constant R.M.S. output voltage appearing between the output lead 14 and ground despite these variations in the amplitude of the input signal. As was indicated previously, one possible use for the circuit 10 might be for supplying nearly constant R.M.S. filament voltage to one or more tubes connected across the output. Any desired number of such tubes may be employed as indicated by the reference numerals 15a, 15b 15n with the filaments of all of the tubes being connected across the output of the circuit 10' between lead 14 and ground.

The lead 14 is connected to one side of the secondary winding 16 of the transformer 13 with the other side of this winding being connected to ground through a silicon controlled rectifier 17 connected across a diode or dry rec tifying means 18. The silicon controlled rectifier, which may be of Type C-35 manufactured by General Electric Company, is poled oppositely to the diode 18. More specifically, the anode 19 of the controlled rectifier and the cathode 20 of the diode 18 are connected directly together and to the lower end of the secondary Winding 16 while the cathode 21 of the controlled rectifier is connected directly to the anode 22 of the diode 18 and to ground. The diode 18 conducts on the negative half cycles of the signal appearing across the secondary winding 16, and since its forward resistance is very low the lower end of the secondary winding is effectively at ground potential during the negative half cycles. The negative half cycles, of course, have no effect on the controlled rectifier 17. On the positive half cycles the diode I8 is effectively an open circuit since its back resistance is very high. The positive half cycles cause the controlled rectifier 17 to conduct only when the potential existing between the gate 23 and cathode 21 is suflicient to cause breakdown. Until the controlled rectifier conducts, it oifers a very high resistance to the A.C. signal and, as a consequence, substantially all of the voltage is dropped thereacross and is deleted from the output. As soon as the controlled rectifier begins to conduct its very low forward resistance elfectively grounds the lower end of the secondary winding and, hence, causes the remaining portion of the positive half cycle to appear across the output. Thus, the output between lead 14 and ground comprises the negative half cycles of the secondary voltage and a portion of the positive half cycles delayed by the time required for the controlled rectifier 17 to fire. As is well known, the breakdown voltage of the controlled rectifier is a function of the potential of the gate electrode 23. Thus, by controlling the potential of the gate electrode, it is possible to control the firing time of the controlled rectifier and, hence, to control the R.M.S. level of the output voltage.

The gate electrode potential is controlled, in accordance with the present invention, by employing an R.M.S. sensing device to detect variations in the R.M.S. voltage appearing between lead 14 and ground and by utilizing the output of this device to alter the potential on the gate electrode in order to compensate for any such variations. More specifically, the R.M.S. sensing device preferably comprises a temperature limited diode 25, for example, a Sylvania 6352 type diode, having a directly heated cathode 26 connected directly across the line 14 and ground. The plates 27 and 28 of the diode 25 are connected directly together and through a load resistor 29 to the positive terminal of a source 30 of DC. voltage. Small variations in the R.M.S. voltage appearing across the cathode of the scnsingdevice .25 cause marked changes in the plate current flowing through resistor 29. Thus, as the R.M.S. voltage across the cathode 26 increases the platecurrent'fiowingthrough the resistor 29 also increases toxproduce a decrease in the potential atlthe =plates.27 and 2/8. The signal appearing across the load resistor 29 is amplified by a conventional emitter follower circuit ineluding a transistor 31 and the amplified signal is applied to the :gate electrode 23. More specifically, the resistor 29 is connected betweenthe base 3'2.and the collector 33 of the transistor 31 in order to provide input drive 'forthe emitter follower. The output of the emitter .followerlis, of course, derived fromstheemitterfi t of 't he transistor 31.

Turning now to the operation ofthe circuit .IOdescr i-bed above, it Will be recognized that any increase in thetRiMS. level of theoutput voltage between :lead 14 and ground is accompanied by an increase inplate current of the diode 25 which, in turn, causes a decrease in the potential of the base 32. The emitter 34 and the gate electrode 23 are thus driven negatively with the result that the firing of the controlled rectifier 17 is delayed. More specifically, the decrease in potential of the ,gateelectrode 23 delays the :fir-ing of the controlled rectifier since the firing point is not reached until a later point of the input voltage waveform. Asa consequence, the latter rectifier does not fire until'a later time in the positive going portion of the cycle. Thus, a greater portion of the positive cycle is deleted so that the R.M.S. level of the output voltage is driven back to the desired level. Any tendency for a decrease in the -R.M.S. voltage across the cathode .26, of course, drives thepotential of the plates 27 and 28 and the base 32 in a positive direction thus increasing the :potential of the gate electrode 23 and causing the controlled rectifier-tofire earlier'in the .positivergoing portion of the cycle. Thus, under the latter conditions the R:M;S. "output voltage is increased to the desired output level. It has been foundthat the circuit described is able to providenegligible changes in R.M.S. output voltage for -input changes :as-great as thirty percent above or below the nominal input level. Moreover, since the sensing device 25, the transistor ,31 and the controlled rectifier17 respond very quickly to changes in applied signals, the overall responsetim'e ofthe circuit lfl'to changes in the level of the input signalis 'verytfast.

In view of the=foregoing discussion it will be observed that the circuit described is effective to accomplish all of the enumerated objects of the invention. While a particular embodiment of the-invention has been :shown, it will be understood, of course, that the invention is not limited thereto since many modifications .maybe made and it is, therefore, contemplated -by-the appended claims to cover any such mOdifications as fall within the true spirit and scope of the invention.

What is claimed as new and desired :to be secured by Letters Patent of theUnited States is:

A circuit for providing a regulated R.M.S. output voltage, said circuit comprising an input transformer having a primary circuit excited by AC. input signals subject to voltage variations and also having a secondary winding for supplying drive signals to a secondary circuit, a pair of output terminals in said secondary circuit, a controlled rectifier connected :in said secondary'circuit betwcen one of said-terminals and one side o f-said secondary winding and having agate electrode and first and secondterminal electrodes, rectifying means connected across said terminal electrodes and poled oppositely to said controlled rectifier so .that .the rectifying means is poled for conduction during one half cycle of the signal appearing across the [secondary .winding while the controlled rectifier is poled :for conduction during the other half cyclewhenever the voltage existing across said terminal electrode becomes sufiicienttto breakdownthe rectifier, the -R.M.S. voltage appearing .across'said .pair of output terminals being inversely proportional to the time required for firing said controlled rectifier during said other half cycle, a voltage sensing:devieecornprising an electron discharge devicehaving a cathode and an anode, said cathode being connected across'said-terminals'and said anode'being connected to a-voltage source todevelo'pa temperature limited DC. control signal in the cathode-anode circuit varying withi'ehan'ges inthe 'R.M-.S. voltage appearing-across said cathode, :andimeans for supplying said control signal to said gate electrode to alter the firing time of the controlled rectifier and, hence, :toc'ontrol the R.M.S. voltage appearing across said output terminals.

References Cited in the file of this patent UNITED 'STATES PATENTS 

